The invention relates to an electrical heating device for hot runner systems, in particular for hot runner manifolds and/or hot runner nozzles, and a method of manufacturing such a heating device.
Electrical heating means for hot runner systems are usually separate component parts with tube-shaped heating elements which are integrated in detachable jackets for peripheral mounting onto flow ducts that commonly are tube-shaped. As disclosed e.g. in DE-U1-295 07 848 or in U.S. Pat. No. 4,558,210, the jackets may be rigid structures whose radii of curvature match the flow duct, additional holding or clamping means being provided for fixing them on the tube periphery in an axial direction. Alternatively, they form flexible heating strips or heating blankets between electrically insulating layers which may have different heat conduction properties and which are fixed onto the tube periphery of the flow duct. EP-B1-0 028 153 provides heat conducting adhesive strips for the purpose, whereas WO 97/03540 employs flexible heating tapes having velcro or other snap fasteners.
Heating devices which in principle are mechanically detachable have the important drawback that heat transition from the heating element to the tube-shaped flow duct is frequently rather inefficient. For compensation it is necessary to enlarge the overall dimensions of the heating device, causing larger heat capacities. The resulting big thermal masses lead to prolonged heat-up and cool-down periods of time, whereby the growth of productivity rates is limited. Moreover, there are problems regarding linear temperature distribution within the walls of the flow duct which rarely feature a constant temperature throughout the length of the flow duct. In the region of the nozzle tip, in particular, sufficient heat transition and thus a sufficient level of temperature can be attained with large expenditures only. This, in turn, affects the entire temperature setting as well as the effort required for controlling means.
It is an object of the invention to overcome these and other disadvantages of the prior art and to create an electrical heating device for hot runner systems providing, between the main hot runner portion and the nozzle, a heat transition and temperature distribution pattern that is generally improved and permits individual precise adjustment. The device is to be designed for easy operation without much effort for control means.
The invention further aims at providing, for hot runner systems, positively and non-positively integrated electrical heating means of compact design which are adapted to be non-detachably mounted onto a flow duct wall such as a mold mass flow tube, a rod, a manifold branch, etc. and which will permanently withstand even extreme mechanical and/or thermal loads.
Another important object of the invention is the development of a method of manufacturing heating devices for hot runner systems, especially for hot runner manifolds and/or hot runner nozzles, requiring a minimum of effort but permitting simple and economical performance.
Principal features of the invention are defined in claims 1 and 19 relating to an electrical heating device and its manufacture, respectively, for use in hot runner systems including manifolds and/or hot runner nozzles with at least one mold mass flow tube associated to a flow duct. The invention provides that at least one insulating dielectric layer is applied by direct coating in an adherent manner onto a wall of the flow tube and is coated by at least one heating layer having heating conductors.
Adherently depositing layers of the heating device results in a permanently fixed connection with the wall of the flow duct and thus in a secure fixing on the hot runner manifold or on the hot runner nozzle. The heating device requires little room owing to the small thickness dimensions achieved through direct coating, whereby in comparison to conventional heating devices, and with almost equal features of performance, extremely compact embodiments can be realized. Moreover, the power density can be distinctly increased since heat is produced and carried off directly at the usually curved surface of the hot runner element to be heated. Together with the direct fixing of the heating device on the flow tube wall in a mechanically non-detachable manner, all of this warrants an always optimal heat transition from the heating layer via the insulating layer onto the wall that is heated most uniformly and precisely. There Is no need for expensive control means which would have to cope with reaction delays caused by thermal masses. The device allows quick and accurate heating and cooling-off again, too, with favorable effects on the entire producing sequence of injection molding.
Another advantage is that the heating device is reliably protected against moisture absorption. Conventional heating devices employing tubular heaters or helix tube cartridges pose, in addition to mounting problems, also insulation problems due to absorption of moisture in a hygroscopic insulating material, as penetrating moisture may cause shortcuts. In order to avoid this, additional control means are required for dewatering by initially operating the heating device under reduced heating power. The heating device of the invention does without that. Rather, it is joined to the flow duct in an absolutely tight and self-captivated manner so that the conventionally necessary effort for mounting and control is completely dispensed with. This has positive effects on the purchase and mounting costs.
Specifically, the at least one insulating layer may be a dielectric layer comprising glass, vitreous ceramics or ceramics. Preferably during the firing process, a pressure pretension is produced within this insulating dielectric layer relative to the flow tube wall, effected by a mismatch in that the linear thermal expansion coefficient (TECDE) of the baked dielectric layer is smaller than the linear thermal expansion coefficient (TECM) of the flow tube wall, the difference between the linear thermal expansion coefficients (TECDExe2x88x92TECM) amounting to at least 5.0 10xe2x88x926 Kxe2x88x921. This further important feature of the invention results in a tension-relief connection between the insulating dielectric layer and the hot runner tube which under operating temperature is exposed to a pulsating interior pressure load technologically caused by the injection molding process. Such load, and the need to heat the flow duct wall up to temperatures between 300xc2x0 C. and 450xc2x0 C. in order to reach and maintain operating temperatures, entail elastic expansions and contractions which are directly transferred to the heating device. The actual degree of deformation will depend on material-bound factors (e.g. elastic modulus) and on technical boundary conditions (operating temperature, tube wall thickness, level of interior pressure). Layers conventionaly applied onto a steel tube will, under the co-influence of the said factors, be freely exposed to varying tensile stresses. The invention, by contrast, avoids or reduces this reliably as the pressure pretension within the dielectric layer will compensate delamination forces occurring under the interior pressure load the magnitude of which varies depending on the respective radii. The heating device as a whole will thus have an extraordinarily good bonding strength on the usually tube-shaped flow tube wall and will permanently withstand even extreme mechanical and thermal loads. Thus optimum production results are always warranted.
The insulating dielectric layer preferably comprises a system of materials including preformed glass, vitreous ceramics or ceramics suitable for wetting, at a predetermined baking temperature, the surface of the flow tube wall which commonly is of metal, said insulating dielectric layer assuming at least partially a crystalline state. The system of materials may include at least one further glass which will not become crystalline under predetermined baking conditions. Additionally or alternatively, the system may comprise at least one compound which is crystalline a priori. By optimizing the proportions of the preformed vitreous and ceramic components of the material system, taking into account their respective TEC increments under the conditions of a firing process, the ceramic dielectric layer will have a linear thermal expansion coefficient (TECDE) in the range between 5xc2x710xe2x88x926 Kxe2x88x921 and 7xc2x710xe2x88x926 Kxe2x88x921.
The at least one insulating dielectric layer may be provided with a gap in a longitudinal direction of the flow tube wall. For protection against outer influences, at least one electrically insulating cover layer may top the heating layer. Moreover, at least one electrically insulating interlayer may be provided between the heating layer and the cover layer or between other pairs of layers.
For purposes of measurement and/or control, it is expedient to provide between the heating layer and the cover layer at least portionwise one further layer e.g. of a PCT (positive temperature coefficient) material whose electrical resistance rises with increasing temperature, this resistor layer forming a thermoelement which may serve for exact control of the melting temperature. If the resistor layer and the heating layer lie in one plane, particularly small overall dimensions are achieved.
Preferably, at least one insulating layer is a ceramic dielectric layer and the or each heating layer includes heat conductors, at least one electrically insulating layer being deposited on top. The insulating dielectric layer, the heating layer, the resistor layer, the interlayer and the cover layer are preferably baked-on foils or baked-on thick-film pastes, and all the layers together may form a layer compound.
Each of the various layers may be separately deposited using foil technology, thick-film technology or screen printing. In the case of thick-film technology, pastes are applied in a round-about printing process. Subsequent baking-on may be performed separately for each of the layers; alternatively, they may be simultaneously baked-on by co-firing. The dielectric layer thus is a baked-on foil or a baked-on thick-film paste whose solid components portion may consist exclusively of a glass that crystallizes in situ at a temperature range above 900xc2x0 C.
If the flow tube consists of a hardened or solidifiable material, such as metal, care is taken that its hardening temperature is not exceeded by the firing temperature of any of the layers. For maintaining a grit structure preformed in the metal, the baking-on of the layers is preferably done by co-firing at temperatures which will not exceed those required for tempering the metal. However, the dielectric layer will also tolerate curing temperatures above firing temperature. The method of manufacture can be optimized in many ways and lends itself to reduction to few process steps.
Such adjustment represents another important aspect of the inventive solution. With a-vantage, the flow tube wall is e.g. inductively heated to hardening temperature. Hardening of the flow tube wall can be performed during at least one of the firing processes, the firing conditions being adjusted to the hardening temperature. A firing temperature between 800xc2x0 C. and 1,100xc2x0 C. is preferable which range corresponds to conventional hardening temperatures for most of the commercial tool steel types for hot working.
For carrying out the method of the invention, inductive heating of the steel tube which is coated with a green ceramic foil or with a thick-film paste not yet baked-on is particularly well suited since in this process, heat transition will start from the inductively heated steel tube and the layer to be baked on will be heated from inside. Consequently, volatile components such as bonding agents and pressure carriers contained in the thick-film paste can escape readily from the glass-ceramic material system that gradually fuses, without inclusion of residual gas. Thus the formation of bubbles is reliably prevented and the grit structure of the layer will be strictly homogeneous.